The present invention is related to an improved method of preparing a solid electrolyte capacitor and an improved capacitor formed thereby. More specifically, the present invention is related to improving the charging time of a capacitor by incorporating work function modifiers in the interfaces between the dielectric and the conductive polymer layer and between adjacent conductive polymer layers.
Solid electrolytic capacitors are widely used throughout the electronics industry. Conductive polymers are widely used in capacitors, solar cells and LED displays with exemplary conductive polymers including polypyrrole, polythiophene and polyaniline. Among them, the most commercially successful conductive polymer is poly (3,4-ethylenedioxythiophene) (PEDT). PEDT can be applied by forming the PEDT polymer in-situ by chemical or electrochemical polymerization or the PEDT can be applied as a PEDT dispersion, preferably with a polyanion, to increase solubility. More particularly, PEDT-polystyrene sulfonic acid (PEDT-PSSA) dispersions have gained a lot of attention due to the high conductivity and good film forming properties. In high voltage applications, solid electrolytic capacitors with a solid electrolyte, formed by conductive PEDT:PSSA based polymer dispersions, give excellent performance compared to conductive PEDT:TSA based polymer cathodes formed in-situ. The structure of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDT:PSS) consists of an insulating PSS layer surrounding doped PEDOT grains. Polystyrene sulfonate is the conjugate base of polystyrene sulfonic acid and the terms are used interchangeable herein depending on the context.
The theoretical charge required to reach rated voltage for a capacitor is given by the formula Q=CV, or taking the first time derivative dQ/dt=I=C dV/dt, where Q is charge, C is capacitance, V is voltage, t is time, and I is current. From these equations, the total charge required to reach the desired voltage can be determined for a capacitor having a given capacitance value. For a constant current, I, using the previous equation charge time (t) is defined by the equation t=CV/I. This is measured using a constant current scanner and a source meter. Using the charge time equation above, the theoretical charge time can be calculated for any given capacitor. In practice, the total charge required by certain capacitors often exceeds this theoretical prediction. Practical consequences of this anomalous charging behavior are that the charging current (dQ/dt) does not fall to low values as quickly as predicted by theory resulting in a slow charging effect. Furthermore, the charging current can exceed theoretically predicted levels when the charge voltage is ramped (dV/dt). This behavior affects measurement of DC leakage current and requires longer times to reach the specified leakage current which can affect capacitor performance in customers' circuits since when DC leakage current is measured, it takes longer than expected for the capacitor's current to fall to levels lower than the specified leakage current for the given application.
Freeman et al. (ECS Journal of solid state science and technology, 2(11)N197-N204(2013)) reported this anomalous charging behavior. They observed a very high anomalous transient current when a short voltage pulse was applied after surface mounting of PEDT:PSS based polymer tantalum capacitors on a circuit board, especially, in dry conditions. They also observed negligible transient current with PEDT:TSA based polymer tantalum capacitors. This anomalous transient current observed with PEDT:PSS did not cause any detectable permanent damage to the dielectric but it decreased with repetition of the voltage pulse as well as after exposure of the capacitor to a humid environment. They also further observed higher charging time at very low temperatures whereas charging time in humid conditions was lower. Authors suggested mobility of the PSS may be contributing to the charging time.
Koch et al., Applied Physics letter 90, 043512, 2007, observed that the work function of PEDT:PSS can be as high as 5.65 eV and that it is strongly reduced by residual water (down to 5.05 eV) as measured by XPS. In addition, uptake of the water is accompanied by pronounced surface composition changes which contribute to changes in work function. Koch et al. suggested that the preferential orientation of PEDT+ and PSS− dipoles with their negative end towards vacuum i.e., for a PSS-rich surface on the surface leads to an increase in work function. Accordingly, they observed lower work function values for samples with lower surface PSS concentration when the PEDT:PSS was treated with moisture in photovoltaic devices.
The experimental observation of lower anomalous current by Freeman et al. with moisture exposure, and the experimental observation of lower surface PSS concentration on moisture treatment by Koch et al. led to Inventor's suggestion that the surface PSS concentration may be playing a larger role in reducing anomalous current or reducing charging time. This understanding lead to efforts focusing on methods to reduce surface PSS concentration or surface charge density.
Mack et al., Application note #52078, Thermo Fischer Scientific, describes a method of measuring and mapping work function using an X-ray photoelectron spectroscopy (XPS). They observed a higher work function in a delaminated interface than in areas that were not delaminated in photovoltaic devices. This suggests that in addition to higher PSS surface concentration, delamination at the interface can also lead to higher work function. This understanding lead to efforts to solve the problems associated with poor charge characteristics by improving the lamination, or decreasing delamination, of adjacent layers.
In spite of the efforts of those of skill in the art it has not been previously realized that the charge time in a capacitor is related to the inherent work function of the conductive polymer layer itself instead of PSS mobility or delamination thereby clarifying the insufficient results from previous efforts. Provided herein is an improved capacitor, and method of making the capacitor, with lowered work function achieved by the use of work function modifiers which minimizes a physical phenomenon not previously considered in capacitors thereby mitigating, and in some cases eliminating, the undesirable charge characteristics and returning capacitors comprising conductive polymer cathodes to, or near, the theoretical charging characteristics which were previously not achieved.